The option to make the deal permanent at the end of the season exists provided that the Gabon international makes a certain number of appearances for the Premier League outfit.

Ndong, 23, recently told Black Cats boss Chris Coleman that he wanted to leave the club with Sunderland not returning to the Premier League at the first time of asking. Indeed, after slumping to relegation last season, Sunderland find themselves immersed in another fight to beat the drop, this time to League One.

An earlier Watford bid for Ndong was rejected but with Sunderland signing Ovie Ejaria joining from Liverpool on loan, the Wearsiders needed to raise funds to cover that fee.

Ndong is suspended for Watford’s match against Chelsea on Monday having been sent off in the 4 – 0 drubbing at the hands of promotion hopefuls Cardiff City a fortnight ago.